Posts Tagged ‘volute’

Last time we discussed how the curved features of a centrifugal pump are key to its functionality. Today we’ll examine a centrifugal pump’s impeller action and see how it creates a volatile environment inside the pump in which cavitation bubbles flourish.

Centrifugal Pump Impeller Action

Inside a centrifugal pump both low and high pressure areas are created, chiefly due to the action of the pump’s spinning impeller. Low pressure is created at the water inlet in a way very similar to what happens when you pull the plug on your bathtub. With the plug removed the drain opens and a tiny whirlpool forms, causing water to get sucked into the plumbing for discharge.

The same thing happens inside a centrifugal pump due to tumultuous internal water movement. The spinning impeller vigorously moves water from inlet to discharge. As water is discharged, a void, or vacuum, is created inside the pump, causing water at the inlet to get sucked inside at low pressure, very much like when you suck liquid through a straw.

As water moves inside the housing, it comes into contact with the rotating impeller itself. This impeller is comprised of multiple spiral curved blades with a volute shape, made to maximize efficient movement of water. They use the power of centrifugal force to create a high pressure environment, and water is flung at high speed towards the pump’s outlet, where it is then discharged.

Next time we’ll see how the coexistence of low and high pressures within the centrifugal pump housing create the problem of cavitation bubbles.

Last time we learned how centrifugal pumps can create a low pressure environment at the pump’s inlet, which can allow water inside the pump to boil at temperatures far lower than normal. Ultimately, this results in the formation of tiny but destructive cavitation bubbles. Today we’ll see how a centrifugal pump’s curved features are key to its functionality.

A Centrifugal Pump’s Curved Features are Key to Functionality

Even a casual glance at a centrifugal pump will disclose its many curved features. As the illustration shows, both the housing and internal impeller blades, are curved. These curves are known as volutes. The volutes’ dimensions are mathematically generated by engineers to facilitate the precise flow of water from inlet to discharge by way of the pump’s impeller blades.

Last week we focused on various types of positive displacement pumps. Today we’ll take a look at centrifugal pumps. See Figure 1.

Figure 1 – A Centrifugal Pump

Just like the positive displacement pumps we talked about last week, centrifugal pumps have rotating parts as well, but that’s where their similarities end. Unlike positive displacement pumps that take “bites” out of liquid before trapping it between moving parts, centrifugal pumps rely on kinetic energy to move liquid in a continuous stream. Kinetic energy is the energy of motion, and in the case of the centrifugal pump kinetic energy is developed by rotating parts within the pump and transferred to the liquid contained within the pump. In other words, the liquid is moved through the pump by means of centrifugal force.

To illustrate this concept, we can tie a rope to the handle of a bucket that has a small hole punched in the bottom. Now, you know what will happen if you fill the bucket with water… There’s a hole in the bucket, Dear Liza, Dear Liza… That’s right, the water will just dribble out of the hole, thanks to gravity. But before we fix the hole as Liza suggests, let’s do an experiment. Pick up the rope and spin the bucket around as fast as you can in a circle. You’ll notice that this rapid spinning creates centrifugal force, resulting in a rather powerful stream of water shooting from the hole. The faster you spin the bucket, the stronger the stream.

When it comes to centrifugal pumps, the idea is basically the same. The objective is to forcefully spin water around in a circle, thus ejecting it from the pump. This is accomplished with a rotating part called an impeller. See Figure 2.

Figure 2 – Cutaway View of a Centrifugal Pump

In our illustration the impeller is attached to a shaft that’s powered by some source of mechanical energy, such as an electric motor. The water enters the pump at the center of the rotating impeller, referred to as the “eye.” The water then slides over the face of the impeller, moving from the center to its edge due to the action of centrifugal force. That force pushes it off the impeller and into the pump housing. You’ll note that the housing has a special shape, called a “volute.” This volute looks a lot like a spiraled snail shell. The shape of the volute helps direct the water coming off the impeller into an opening in the side of the pump where it is discharged. The faster the pump impeller rotates, the more kinetic energy the water picks up from the impeller.

This ends our discussion on pumps. Next time, we’ll move on to a new topic of discussion, braking systems.